1. Field of the Invention
The present invention relates to a copying apparatus and particularly to a copying apparatus in which a toner image based on an image of an original is transferred onto copy paper and fixed.
2. Description of the Related Art
A copying apparatus is provided with a fixing device for fixing a toner image transferred on copy paper.
FIGS. 11A and 11B are enlarged views of a fixing roller portion.
Referring to those figures, the fixing device comprises a heating roller 5a containing a heater for example, a pressing roller 5b which rotates in contact with the heating roller by applying pressure thereto, and separation claws 150a and 150b for separating copy paper P passing through the fixing roller portion, from the rollers 5a and 5b. Normally, when the copy paper P passes between the rollers 5a and 5b, toner of a transferred image FP is pressed and thermally fused on the copy paper and fixed thereon. The copy paper P having the image fixed thereon is separated and discharged from the rollers 50a and 50b by means of the separation claws 150a and 150b.
However, since the melted toner has some viscosity, if a black solid image exists in a leading edge portion of the copy paper, the copy paper would wind about the heating roller 5a.
More specifically, if the transferred image FP is formed as far as the leading edge portion of the copy paper P as shown in FIG. 11B, the copy paper P passes through the rollers with the leading edge of the copy paper P adhering to the heating roller 5a due to the melted toner. On the other hand, there is a certain gap between the separation claws 150a and 150b and the rollers 5a and 5b, respectively, so that the rollers may rotate smoothly. Accordingly, the copy paper P is not separated by the separation claw 150a and it winds about the heating roller 5a.
Particularly, in a copying apparatus having a duplex copy function of executing copy on both surfaces of copy paper, two copy operations are required for one sheet of copy paper. More specifically, copy paper fed from a feed opening is transported to an intermediate tray and stored temporarily therein after the first copy operation and then it is refed from the intermediate tray so that the second copy operation is performed.
Accordingly, in the case of duplex copying, one sheet of copy paper passes through the fixing device twice and the leading and rear edges of the copy paper passing through the fixing device are interchanged. In consequence, in the case of duplex copying, toner at the leading edge and that at the rear edge of the copy paper cause the paper to wind about the roller and also cause fouling of the rollers or the copy paper.